The present invention relates to a soldering apparatus for spraying a molten solder such as soft solder from a nozzle onto the rear surface of a printed circuit board.
A soldering apparatus shown in U.S. Pat. No. 3,797,724 has been proposed which solders leads of electric elements mounted on a printed circuit board to a circuit pattern formed on the rear surface of the board.
As shown in FIG. 1, this apparatus has a solder tank 1 containing a molten solder 2 such as soft solder. A flow channel 6 extends from one end to the other end of the tank 1. The molten solder 2 is caused to flow in the channel 6, from one end to the other end of the tank 1 by an electromagnetic pump 3 surrounding part of the channel 6. The molten solder 2 reaches a slit nozzle 4 projecting upward from the other end of the tank 1 and is sprayed therefrom in the form of a thin film. A printed circuit board 5 to be soldered is conveyed above the nozzle 4 in the direction indicated by arrow A, and its bottom surface is uniformly sprayed with the solder. In this manner, the leads of the electric elements projecting from the rear surface of the printed circuit board 5 are soldered to the circuit pattern formed on the rear surface of the board 5.
Referring to FIG. 2, the electromagnetic pump 3 has a C shaped iron core 7 whose forked ends sandwich channel 6 so as to generate a magnetic field perpendicular thereto, and a coil 8 wound around the iron core 7. A pair of electrodes 9 are arranged perpendicularly to the channel 6 and to the magnetic field of the iron core 7. Therefore, a current flows through the molten solder within the channel 6 in a direction perpendicular to the channel 6 and to the magnetic field of the iron core 7. The directions of the magnetic field of the iron core 7 and the current flowing between the electrodes 9 are so selected that a force acts in the direction indicated by arrow B in FIG. 2 to cause the molten solder 2 to flow in the channel 6 toward the nozzle 4.
However, since the specific gravity of the molten solder such as soft solder is relatively great, the solder may not be sprayed from the nozzle 4 unless a relatively great force acts on the molten solder in the channel 6. In the conventional soldering apparatus, that portion of the channel 6 on which the magnetic field generated by the iron core 7 can act is limited. For this reason, the intensity of the magnetic field to be generated by the iron core 7 must be increased in order to obtain a greater force. This requires a bigger iron core 7 and a bigger coil 8. Since electrodes 9 are also required, the electromagnetic pump 3 becomes bulky. Furthermore, a current flows in that wall portion of the channel 6 which contacts the electrodes 9. For this reason, these wall portions corrode and must be repaired frequently.
In addition, the depth of the tank 1 is greater than the width of the printed circuit board 5 to be soldered, resulting in a larger tank 1 and a greater amount of molten solder to be stored therein. However, since only a small amount of solder is applied to the rear surface of each printed circuit board 5, most of the solder in the tank 1 remains molten therein for a long period of time, and is oxidized upon contact with the air. A greater amount of solder to be stored in the tank 1 also requires a larger heater for heating it. It also takes a longer period of time before the solder is molten and ready to be sprayed when the solder is heated from the solid phase.
The solid solder must begin melting from the interior of the tank 1 toward the channel 6. However, since the channel 6 is spaced apart from the tank 1, it takes a long period of time for the solder in the channel 6 to melt.
In a conventional soldering apparatus of the configuration described above, the coil 8 is not cooled directly. Therefore, the electromagnetic pump 3 cannot be cooled satisfactorily.